A protection tube for an optical fiber and the like is formed by combining three deformed steel wires, which have a hollow-fan shape (that is, cut up a doughnut into fan shapes) of cross section, in a cylindrical shape. An optical fiber is protected from an external force by being inserted into the protection tube, and the protection tube serves as a pressure-resistant layer. To decrease the installation costs of a cable, there has been a demand for the weight reduction of the protection tube or the pressure-resistant layer, and thus far, there have been proposals regarding, for example, a deformed steel wire having a high tensile strength as disclosed in Patent Documents 1 to 4. The deformed steel wire is manufactured by drawing a wire rod, and then deforming the wire rod so as to form a substantially hollow-fan shape of cross section. Therefore, for example, a high-strength steel wire satisfying both conditions of cold workability and high strength is proposed in Patent Document 5. Here, the wire rod refers to a material of a steel wire. In addition, the steel wire is a collective term for a deformed steel wire and steel wires other than the deformed steel wire.
To satisfy the cold workability and high strength of a steel wire, an increase in the cementite fraction, the strengthening of ferrite by a solid solution of Si or the like, and a decrease in the lamella spacing of pearlite are effective. The above-described steel wire is manufactured by, for example, hot rolling a steel piece, which includes a great amount of C, produced by adding Si, Cr, and the like as necessary, so as to produce a wire rod, furthermore, heating the wire rod up to an austenite region, then, cooling the wire rod, and carrying out a patenting treatment for isothermal transformation after the cooling.
A deformed steel wire of the related art having a high tensile strength satisfied both conditions of high strength and high ductility using the metallographic structure of a steel wire that had been subjected to hot rolling or patenting treatment. In addition, regarding a drawing process or a deforming process for manufacturing a deformed steel wire satisfying both conditions of high strength and high ductility, for example, methods for adjusting the reduced cross-sectional area or the caliber shape of a roller to prevent wire rapture have been proposed in Patent Documents 2 and 3.
However, for example, the technology that adjusts the reduced cross-sectional area or the caliber shape of a roller, which is disclosed in Patent Documents 2 and 3, is effective to ensure the tensile strength of a deformed steel wire, but it cannot be said that a deformed steel wire obtained using this technology has excellent low-cycle fatigue properties. In addition, for example, in Patent Document 1, there is no regulation regarding the texture or the dimensional accuracy, and thus it is difficult to satisfy both conditions of the uniformity of the tensile strength and excellent low-cycle fatigue properties, that is, durability.
Furthermore, when a power cable or a communication cable is installed in the ocean from a vessel, the cable is affected by bad weather, ocean waves, and the like within a short period of time during which the cable reaches the ocean floor, particularly, the deep ocean floor and is stabilized. Therefore, there is a case in which great tension is repeatedly applied to the cable due to water pressure or tidal force, and the breakage of the cable due to the low-cycle fatigue is concerned. Therefore, the deformed steel wire protecting an optical fiber and the like is required to have low-cycle fatigue properties.